Abstract Black carbon (BC) aerosol critically influences climate and haze, yet the mechanisms driving its coating and light absorption enhancement remain poorly understood. Here, we show that BC in Beijing is coated by ammonium nitrate via photochemistry. Laboratory experiments, quantum calculations, and machine learning reveal a new pathway: hydroxyl groups on photoaged BC surfaces promote nitric acid formation in the presence of NO2, which then neutralizes with ammonia to produce ammonium nitrate. Mie simulations demonstrate that this coating drives enhanced light absorption alongside haze intensification. Our findings bridge a key gap between laboratory studies and field observations, elucidating the aging and coating mechanisms of BC in a typical megacity.